1. Field of the Invention
This invention relates to a method and apparatus used for heat treatment of steel.
2. Prior Art
This invention is an improvement of the applicant's patent application filed in Japan, No. 47-8916, entitled "Heat Treatment Method for High Alloy Steel" and Japanese application No. 47-45776, entitled "Hardening Process for High Alloy Steels". In patent application No. 47-8916, the gas constituents are limited to the following: carbon monoxide 0.2 to 1.0%, hydrogen 0.5 to 2.0%, the remainder being nitrogen gas. All percentages per volume percent. In cases where a steel or an iron article containing large quantities of oxide is heated in a process wherein simultaneous heat-hardening and soldering takes place, the reducing power of the above-identified gas constitutes the atmosphere of the furnace and has been found to be insufficient to produce the desired surface characteristics. This has lead to poor results when such articles are soldered together. This invention, and more specifically, the atmosphere specifically set forth herein, permits simultaneous soldering and heat-hardening of iron and steel even those articles with an outside coating adhering thereto by the use of an atmosphere containing 0.5 to 1.5% carbon monoxide, 0.5 to 2.5% hydrogen, and the remainder of the atmosphere containing nitrogen with a minute amount of activated carbon therein, again, all percentages being volume percent.
Patent application No. 47-45776 is concerned with atmospheric cooling as is the present application. In that application the method to cool the article involved the consumption of large quantities of nitrogen gas. This represents an economic problem as such cooling process is relatively expensive and time consuming. This invention further includes a process whereby the atmospheric gas of the furnace is passed through a heat exchanger which thereby cools it and returns the gas to the furnace, which gas as described hereinabove is almost completely nitrogen. The gas is then reused as a quenching gas.
The present invention, therefore, is directed to a method and apparatus which are further improvements in the art of heat treatment of iron and steel articles.